The Carbon Cycle 2I. Introduction: Changes to Global C Cycle

(Ch. 15)II. C-cycle overview: pools & fluxes (Ch. 6)III. Controls on GPP (Ch. 5)IV. Controls on NPP (Ch. 6)V. Controls on NEP (Ch. 6)

Powerpoint modified from Harte & Hungate (http://www2.for.nau.edu/courses/hart/for479/notes.htm) and Chapin

(http://www.faculty.uaf.edu/fffsc/)

The Carbon CycleIII. Controls on GPP (Ch. 5)

A. Introduction 1. Photosynthesis 2. Proximal & distal controls

B. Photosynthesis overview 1. Light-harvesting & C-fixation rxns. 2. C3, C4 & CAM Ps

C. Controls on photosynthesis 1. Principle of environmental control 2. Limiting factors

a. Light: i. Leaf-level; ii. Canopy-levelb. CO2 : i. Leaf-level; ii. Canopy-levelc. N : i. Leaf-level; ii. Canopy-leveld. Water : i. Leaf-level; ii. Canopy-levele. Temp : i. Leaf-level; ii. Canopy-level

Powerpoint modified from Harte & Hungate (http://www2.for.nau.edu/courses/hart/for479/notes.htm) and Chapin

(http://www.faculty.uaf.edu/fffsc/)

I. Introduction

6 CO2

C6H12O6

organic carbon

energy6 H2O

+

6 O2

Photosynthesis = process governing energy capture by ecosystems, and thus the entry of organic (fixed) carbon into the biosphere

Controls on GPP:Across ecosystems – LAI, N, growing season lengthWithin ecosystems (daily, seasonal) – light, temp, nutrients

5.1

B.Photosynthesis overview

PhotosynthesisTwo sets of reactions:

Light harvesting - converts light energy to chemical energy (ATP, NADPH) - splits H2O and produces O2 as byproduct

Carbon fixing - uses ATP and NADPH from light reactions to fix CO2 into sugar

5.3

Rubisco

CO2 enters leaf via diffusion

Stomata (plural), stoma (singular): biggest and most variable component of

diffusion barrier (resistance)

Water on cell surface evaporates, diffusing out through the stomata, leaf boundary layer, and into the bulk atmosphere: transpirationStomata: carbon gain, water loss – inevitable trade-off

CO2 H2O

B.2. Three major photosynthetic

pathways•C3 photosynthesis

–“Normal” photosynthesis, 85% of plants

•C4 photosynthesis–Spatial separation of 2 carbon fixation paths

•CAM (Crassulacean Acid Metabolism)

–Temporal separation of 2 carbon fixation paths

C3 Photosynthesis

http://www.digitalfrog.com/resources/archives/leaf.jpg

C4 Photosynthesis

C4 Distribution

CAM Photosynt

hesis

3 photosynthetic pathways• See pp. 102-105, including Box 5.1

1. How does C3 differ from C4 in terms of initial fixation enzyme, site of initial fixation, use of Calvin cycle?

2. How does C4 differ from CAM?3. How do C4 and CAM reduce water loss

and photorespiration? 4. What tradeoffs are inherent in C4 &

CAM?Bottom line: C4 and CAM reduce water loss and reduce photorespiration because PEP carboxylase has a higher affinity for CO2 and no affinity for O2.

These adaptations are most important in hot, dry environments.

C.Controls on Photosynthesis

Net Ps = C-fixation – mitochondrial resp – photoresp

(not to be confused with NPP)

1.Basic principle of environmental control

CO2 response curve of photosynthesis:

1. Net Ps2. Compensation point3. CO2 diffusion4. Biochem limits: light-harvesting, Rubisco

(N), RuBP (P)

5.6

5.6

Where is leaf most efficiently allocating its resources for C gain?

Plants adjust photosynthetic “machinery” and internal CO2 to operate at the balance point

5.6

Basic Principle of Environmental Control: Equalize physical and biochemical limitations of photosynthesis

5.6

HOW?• Shade vs. sun?• Fertile vs. infertile soils?• Wet vs. dry environments?

C. Controls on photosynthesis

• Most leaf-level controls still function in entire canopies

• Leaves at top of canopy carry out most photosynthesis– Receive most light– Youngest, most N-rich leaves

C. Controls on photosynthesis

2. Limiting factorsa. Light

i. Leaf-levelPoint 1: Light can vary greatly at time scales of tenths of

seconds to minutes to days to seasons.

5.7

- But, light doesn’t account for differences across ecosystems.

- Large control of temporal variation in photosynthesis within ecosystems

Point 2. Light response curve of photosynthesis

6 points: Isat, Psmax, LCP, Ps at low light, decline (photo-ox.), LUE(draw)

Point 3.Plants have a variety of mechanisms for adjusting to

variation in light• Acclimation (physiological adjustment)

– Sun leaves• More cell layers (draw mesophyll)• Higher photosynthetic capacity

– Shade leaves • thinner, more surface area/g• More light-harvesting pigments

• Adaptation (genetic changes)– Mechanisms same as for acclimation– Traits persist even when plants grown in

similar conditions

Point 3. Mechanisms of adjusting to variation in

light• Other neat tricks

– Maximize/minimize leaf area

• More leaves• Thin leaves or cylindrical

leaves

– Leaf angle– Leaf movements– Efficient use of sun flecks

Oxalis oregona

Adenostoma fasciculatum (chamise)

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

5.9

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

1. Isat

5.9

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

2. Psmax

5.9

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

3. Light compensation point

5.9

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

4. Rate of Ps at low light

5.9

AB

C

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

5. Light level for photo-oxidation

5.9

Adaptation/Acclimation result in different light response curves for 5/6 of the components we

discussed

6. Quantum yield at low light stays the same

5.9

These adaptations are the same for1. Leaves on the same individual in different environments2. Individuals of the same species in different environments

3. Different species specifically adapted to different environments

5.9

ii. Light – Canopy level controlsPoint 1. Multiple species increase range of light levels over which

light use efficiency remains constant

5.9

Canopy processes increase range of light intensities over which LUE is constant

Point 2. Vegetation maintains relatively

constant LUE• Leaf level regulation

– Balance biochemical and physical limitations to photosynthesis

• Canopy level regulation– Maintain highest Ps capacity at top of

canopy– Shed leaves that don’t maintain

positive carbon balance

Point 3. Leaf area • Leaf area determines both amount of

light intercepted and light environment in the canopy.

• Leaf area responds to availability of soil resources (more soil resources, more growth)

• Light declines exponentially within canopy.

• LAI ~1-8 m2 leaf/m2 ground• Projected vs. total LAI – what’s the diff?

C. Controls on photosynthesis

2. Limiting factorsb. CO2 – see book

c. Nitrogen

5.1

Soil resources (nutrients, water) influence both - amount of plant growth (leaf area) - amount of N in photosynthetic machinery in

those leaves.

i. Leaf-levelPoint 1. Leaf nitrogen determines photosynthetic capacity

Why?

Point 2. Stomatal conductance adjusts to match photosynthetic capacity

(or vice versa)

5.11

Point 3. Leaf longevity is a major factor determining photosynthetic capacity per gram tissue

Inevitable tradeoff between photosynthesis and leaf longevity

Long-lived leaves contain lots of non-photosynthetic compounds

Herbivore protectionDesiccation resistant

5.12

SLA is a good predictor of photosynthetic capacity

5.14

Suite of traits that influence carbon gain depends on

availability of soil resources

• Leaf longevity• Leaf nitrogen concentration• Photosynthetic capacity• Growth rate

ii. Canopy-levelPoint 1. Fast-growing plants have high photosynthetic

rates

and more growth more leaf area more growth …

Point 2. High soil resource availability increases competition for light

• More growth, more leaves, decreased light near the ground.

• Fertile soils, high water availability select for plants with high growth rates (change in plant functional types).

• What allocation strategies might help a plant grow fast?

5.1

Communities with high levels of soil resources typically support intrinsically faster growing species.

Differences among ecosystems in LAI are a major control on GPP (and NPP)

5.20

Schlesinger 1997

Carbon gain estimated from satellites

NDVI = (NIR-VIS)(NIR+VIS)

NDVI: Normalized difference vegetation indexNIR: Near-infrared radiationVIS: Visible radiation

Schlesinger 1997

LANDSAT – local AVHRR – regional-global

LAI correlates with NDVI

Schlesinger 1997

d.Water limitationi. Leaf-level

– Short-term response: reduce stomatal conductance (reduces LUE)

5.4

d.Water limitationmedium-term response: reduce leaf

area (reduces surface area for water loss, maintains high LUE)

Tropical dry forest, Mexico

d.Water limitation• Long-term response

(adaptation): – reduce light absorption (smaller

leaves, inclined leaves, hairy leaves)– C4, CAM photosynthesis

http://www.sci.sdsu.edu/plants/sdpls/plants/Adenostoma_fasciculatum.html

d. Water ii. Canopy-level

• We’ll talk about water and temp together.

e. Temperature i. Leaf-level

Point 1. Plants acclimate to typical temps on sunny days.- both low and high temperature restrictions on Ps.

Point 2. Different adaptations in different environments

- Increased photosyn. capacity in cold environ.- High ET in warm wet environments- Small leaf size in warm dry environments

ii. Canopy-levelPoint 1. Plant adaptations reduce differences among ecosystems directly resulting from temperature within the growing season.

ii. Canopy-levelPoint 2. Differences in temp and water availability are major controls on growing season length, which has a strong effect on annual GPP.

f. Response to pollutants(see book)

• Damages photosynthetic machinery

• Reduces photosynthetic capacity

• Plants reduce stomatal conductance

Main points about photosynthesis

• Balance biochemical and physical limitations

• Match photosynthetic potential to soil resources

• Adjust leaf area to maintain constant LUE

Major controls over GPP – across ecosystems

• Quantity of leaf area– May be reduced by herbivores and

pathogens

• Length of photosynthetic season• Photosynthetic rate of individual

leaves– Photosynthetic capacity– Environmental stress that alters

stomatal conductance

Relationship of NDVI to ecosystem carbon gain

(Satellite estimate)

(Measured light absorbed)